The present invention relates to a thermal transfer printing apparatus, and, more particularly, to a thermal transfer printing apparatus in which a pixel is formed by a plurality of dots in a matrix form, and can be printed in a halftone mode, or with gradation or gray levels.
In a thermal transfer printing apparatus, a thermal head (printing head) is urged against printing paper through an ink film (normally having a ribbon-shape), and an ink on the ink film is melted by heat generated when heating resistors constituting the thermal head are energized. Thus, the melted ink is transferred to the printing paper so as to form a dot image corresponding to the energized resistors. In this apparatus, each dot can only be binary-controlled as to whether or not the ink is transferred. Therefore, in order to print a halftone image such as a picture, a so-called binary area modulation method is generally adopted. In this method, a pixel must correspond to a plurality of dots in a matrix form. The number of dots which are energized and subjected to ink transferring, however, changes in accordance with the density of a pixel. A DITHER method, a micro-font method or the like are well known as binary area modulation methods.
However, the number of levels able to be represented by this area modulation method is limited. When a pixel has an n.times.n dot matrix configuration, the number of levels expressed is n.sup.2 +1, including 0 level (the level of the printing paper). For example, in the case of a 4.times.4 dot matrix, 17 levels are provided. In general, a color image requires a resolution of 4 dots/mm or higher, and each color component requires 64 gray levels or more. In order to satisfy these requirements with the above-mentioned area modulation method, a pixel must be configurated by an 8.times.8 dot matrix, and a thermal head having a resolution of 32 dots/mm or higher is needed. Although a thermal head having a resolution of 16 dots/mm has been developed, it is difficult to realize one having a resolution of 32 dots/mm or higher. For this reason, in this area modulation method, requirements for the number of gray levels and resolution cannot be satisfied, and it is impossible to perform halftone printing having a gradation that in both smooth and fine.
In a thermal transfer printing apparatus, a portion designated to remain white in a high or medium density region may be colored due to heat pile-up of the thermal head. In contrast to this, an ink may not be transferred due to an insufficient amount of heat in a low density region. For this reason, a halftone image cannot be printed precisely.